JP-A-11-180249 discloses a conventional protection system against lateral collision that is fit on a vehicle chassis. This protection system has acceleration sensors for sensing the lateral force acting on the vehicle chassis. If the output from each of the acceleration sensors exceeds a threshold, the protection system determines that the vehicle had a lateral collision. This protection system then actuates protecting devices of the vehicle to protect the passengers in the vehicle. As a general rule, a low-frequency component of the output from each of the acceleration sensors is extracted through a low-pass filter or an integration circuit. The amplitude of the low-frequency component is compared by a comparison circuit so that malfunctions due to high-frequency noises can be suppressed.
In this system, the acceleration sensors are fit to the right and left side walls of the vehicle chassis as side wall sensors. If another vehicle collides with one of the side walls, a great impact force acts on the associated sensor mounted near the side wall. This sensor then outputs a voltage proportional to the impact force. This output voltage triggers to actuate the airbag or the like installed in the vehicle chassis. Each of the acceleration sensors (side wall sensors) mainly senses the plastic or elastic deformation of the adjacent side wall due to a lateral collision of the vehicle chassis. Each sidewall sensor also senses the movement of the whole vehicle chassis due to the lateral collision.
The side wall sensors become less sensitive to the impact force as located away from the point on the side wall where the impact force acts. Accordingly, two such sensors should be fit to a front portion and a rear portion of each of the right and left side walls (4 sensors in total). More preferably, three such sensors should be fit to a front portion, a middle portion and a rear portion of each side wall (6 sensors in total). However, if the driver strongly closes a door of the vehicle, impact force that is not so weak acts on the side wall sensors, which may then cause the airbag or the like to malfunction.
In order to solve this problem, it is proposed that an additional acceleration sensor be fit in the vehicle dashboard and near the center of the vehicle, which is less influenced by impact force than the side walls when the doors are opened and closed. The impact force developed by a lateral collision of the vehicle is damped and delayed by the deformation and/or displacement of the vehicle chassis before transmitted to this central sensor. Accordingly, by using the output from the central sensor, it is possible to exclude relatively small impulses, such as the impulses developed when the doors are closed strongly, and to sense only the great impulses developed by the lateral collision. It is also proposed that the central sensor and the side wall sensors be fit in the vehicle chassis to improve the sensing accuracy.
However, the outputs from the acceleration sensors for sensing lateral collisions are influenced by not only the collision impact but also the lateral movement of the vehicle chassis due to the lateral collision etc. Depending on which longitudinal position on the vehicle the lateral collision occurs, this influence may reduce the impact forces sensed by the acceleration sensors. Consequently, the passenger protection system may not be able to be properly actuated.
For example, if a lateral collision occurs at a rear portion of the vehicle on a road surface having a low coefficient of friction, the vehicle is rotated angularly laterally in the lateral collision direction and may be rotated angularly to a considerable degree. If the center of rotation (spinning) is rear from the acceleration sensors, forces (acceleration) act on the sensors in the direction reverse to the collision direction. After all, the rotation makes the outputs from the acceleration sensors lower than if the center of rotation is not rear from the sensors.
When a lateral collision occurs with the vehicle, the center of rotation of the vehicle is determined by the center of inertia mass (the longitudinal center of gravity) of the vehicle and the positions of the four tires, which resist the rotation of the vehicle. In general, when a lateral collision occurs with the rear portion of the vehicle, the center of rotation is positioned longitudinally midway between the front and rear tires.